PROTOZOA AS CELLS 31 



injector canals as the main vacuole expands. Schneider suggests 

 that systole of the main vacuole is initiated by contraction of the 

 fibers seen in its walls. These would first close the injector canals 

 and then exert pressure on the vacuole contents, forcing rupture 

 of the septum covering the exit pore. Upon release of its contents, 

 the medial wall of the vacuole is pressed toward the lumen, 

 possibly by elastic reaction of the surrounding cytoplasm to com- 

 pression suffered during diastole. The septum over the exit canal 

 is repaired during the resting phase following systole, while the 

 radial canals are filling and the main vacuole remains collapsed. 



This hypothesis would appear to be mechanically plausible. 

 The arrangement of the 20-m/x fibrils along the walls of the system 

 is consistent with the assumption that they are contractile, and 

 similar fibrils have been observed in other contractile organelles 

 among the protozoa (see descriptions of Pjrsonympha, Chapter 5, 

 and Stentor, Chapter 6), as well as in association with contractile 

 vacuole pores of other ciliates described above. 



If, as several studies indicate, a membranous septum closes the 

 exit canal and must undergo rupture and repair with each pulsating 

 cycle (that is, every few seconds), this constitutes a rather remark- 

 able example of membrane activity. 



In all of these studies, the cytoplasmic zone in which segregation 

 of water and resorption of solutes almost certainly takes place is 

 seen to be occupied by tubules or vesicles providing a large, and 

 in the more extreme cases enormous, membrane surface area. 

 Linkage of these tubules with endoplasmic reticulum may be 

 significant if the latter system can be demonstrated to function in 

 water transport elsewhere in the cell; the contractile vacuole 

 apparatus occupies, after all, only a relatively small part of the 

 total cell volume. In fact, this picture, as Schneider points out, 

 constitutes one of the best parcels of morphological evidence that 

 such may be the case. Needless to -say, proof of any of these 

 assumptions will require more than morphological evidence alone. 

 It would be interesting to learn whether contractile vacuoles with 

 a conspicuous cortex of packed tubules are more active in their 

 output than those with relatively fewer surrounding vesicles. 



One question that is particularly puzzling is how the vacuoles 

 or radial canals that receive water from the surrounding sponge 

 swell as much as they do. The influx of fluid during diastole must 



